1. Field of the Invention
This invention relates to a blend of low density polyethylene and crystalline polypropylene and fibers produced therefrom. In particular, the invention concerns an improved blend that can be melt-spun at high temperature and high speed into fibers which are particularly useful as binder fibers for nonwoven fabrics.
2. Description of the Prior Art
Nonwoven fabrics which contain fibers having different melting temperatures are known in the art. The fibers with the lower melting temperature act as an adhesive agent which bonds the higher melting-temperature fibers to each other. The lower melting-temperature fibers are referred to as "binder" fibers. In the manufacture of many nonwoven webs which include binder fibers, the web is compressed and heated to a temperature which causes the binder fibers to melt and bond the other fibers to each other. Temperatures which do not detrimentally affect the tensile characteristics of the non-binder fibers are desired for the binding operation.
Polyethylene, because of its relatively low melting temperature and other desirable characteristics, has been considered for use as binder fibers for nonwoven fabrics, especially for those made of polypropylene filaments. However, polyethylene generally has been found to be unsatisfactory for binder fibers because of its poor spinnability. The melt-spinning of polyethylene into fine filaments at high speed and high temperature has generally been unsatisfactory. Yet, such high temperature, high speed melt-spinning is highly desirable for successful commercial production of polyethylene binder fibers. Accordingly, it is an object of this invention to provide a composition that can be melt-spun at high speed and high temperature into binder fibers that would have melting characteristics similar to those of polyethylene.
Many blends of polyethylene and polypropylene are known in the art. For example, Plastics and Polymers, Vol. 40, No. 147, pages 142-152, "Parameters Affecting Processing of Polymers and Polymer Blends," by W. H. Skoroszewski, (June, 1972) discloses blends of polyethylene in polypropylene, with the polypropylene being the major component and high or low density polyethylene being the minor component. Other blends of linear (i.e., high density) polyethylene and polypropylene are disclosed, for example, in Journal of Applied Polymer Science, Vol. 26, 3515-3521, "Drawing Behavior of Polyethylene-Polypropylene Blends" by P. Robson et al. (1981).
Blends of low density polyethylene and isotactic polypropylene are known for various purposes. For example, Maloney et al., U.S. Pat. No. 3,355,520, suggests that branched polyethylene in which is incorporated polypropylene amounting to 3.1 to 5.3% (by weight of the polyethylene) improves the wire-coating, film, and bottle-forming characteristics of the polymer. The examples of Maloney et al. disclose branched polyethylene of 0.922 and 0.970 g/cm.sup.3 density and 0.17 and 0.07 melt index in which is incorporated up to 11.0% of polypropylene of 0.905 g/cm.sup.3 density and 0.78 melt flow rate.